1. Field of Invention
This invention relates to a suspension system for wheeled vehicles, particularly for two, three, or four-wheeled vehicles, more particularly for bicycles. More specifically, the invention relates to an improved pivotless rear suspension system for a bicycle or other vehicles.
2. Background Art
All references disclosed herein are incorporated by reference. Bicycle suspension systems are so called because they suspend the weight of the rider and bicycle by springs or the like acting upon the wheels, thereby providing isolation from bumps in the road or trail. During the evolution of the bicycle, there have been many attempts to provide a simple and effective rear suspension system. A resurgence of activity in this field has been prompted by the current popularity of dirt road and trail riding, commonly called mountain biking. A well-designed bicycle suspension can improve control and safety, as well as comfort, by keeping the tires in contact with the ground on rough terrain. Most rear suspension systems used to date have been problematic, for reasons explained below.
Problems with Existing Rear Suspensions
The vast majority of suspension systems rely on one or more pivots. Pivots have inherent problems: they are costly to manufacture, they tend to develop squeaks and looseness requiring maintenance and adjustment, they wear out, and they concentrate the suspended movement to a single point rather than distributing the movement over a geometric range.
Previous attempts to utilize pivotless suspensions proved to be too flexible in lateral and torsional loadings, while offering very limited travel.
Many prior-art bicycles use an oil-damped air-sprung shock absorber. They tend to be long, they have many pressurized seals which can leak, and they are not good at isolating the rider from high-frequency trail irregularities.
Prior-Art Bicycles with Pivots
Perhaps the simplest rear suspension is to mount the rear wheel on a swingarm which is pivotably mounted to the rest of the bicycle. A logical place to pivot the swingarm on such a bicycle is at or near the pedal crankshaft so chain tension doesn""t vary as the rear wheel swings. An early example of this type of suspension is seen in U.S. Pat. No. 398,158 to Jeffery (1889).
With a modem linkage suspension, such as shown in U.S. Pat. No. 5,509,679 to Leitner (1996), an extra pair of pivots between the rear of the chainstay and the axle of the rear wheel allows increased travel without producing pedaling-induced bobbing. However this type of system tends to be prone to noise and wear, due to contamination of the pivots with dirt, compounded by the high number of pivots.
Prior-Art Pivotless Bicycles
In attempts to eliminate the problems associated with pivots, several suspension designs have utilized the inherent flex characteristics of frame members to achieve suspension action.
Early examples of pivotless suspensions can be seen in the Worth Spring Frame Safety Bicycle, sold circa 1890 by the Chicago Bicycle Company, and in a similar suspension disclosed in U.S. Pat. No. 621,942 to Pierce (1899). Both of these bicycles use full-length leaf springs, of constant rectangular cross section, as chainstay members. The problem with these designs is that the stays are narrow, hence not sufficiently capable of resisting lateral and torsional loading. The resulting excessive lateral flex precludes good handling.
An attempt to resist lateral flex is disclosed in U.S. Pat. No. 618,627 to Travis (1899) which uses a wider plate, but only for the portion of the chainstay in front of the tire. While this helps lateral stiffness, the short length of the flexible plate allows only limited angulation at the front of the chainstay, and cannot accommodate sufficient angulation between the rear of the chainstays and the seatstays. Finlay et al., U.S. Pat. No. 2,283,671 (1949), take a different approach, using flattened tubing for the rearward portion of the chainstays. But again, the short length of the flexible portion reduces the travel which can be attained with this design.
More recently U.S. Pat. No. 5,098,114 to Jones (1992) discloses a pivotless suspension using tubular chainstays made from a highly flexible material such as titanium. A similar example is sold under the mark YBBeat by Moots of Steamboat Springs, Colo. The problem with these designs is that it is impossible to get a round tube to flex vertically without also flexing laterally by a similar amount. Consequently, these bicycles suffer from too much lateral flex, and too little suspension travel. Another problem is that the shock absorber load also goes off-axis under large bumps, which causes binding and wear of the shock absorber to occur.
U.S. Pat. No. 5,405,159 to Klein et al. (1995) discloses a four-bar-linkage suspension with four xe2x80x9cx-pivotxe2x80x9d type flexures on the chainstays, plus additional mechanical pivots. His proposed configuration lacks adequate bracing for lateral loads, appears impractical to fabricate, and it is not completely pivotless.
Completely pivotless designs are disclosed in U.S. Pat. Nos. 4,856,801 to Hollingsworth (1989) and 5,533,742 to Peart (1996). These references disclose bicycles with the chainstay cantilevered off the main frame. Because of the absence of seatstays, all bicycles of this type suffer from extremely poor torsional stiffness due to independent vertical motion of the left and right ends of the chainstay.
All of the above pivotless systems typically suffer from lateral flex and/or limited travel, and other problems as outlined above. The Applicant discloses both pivoted and pivotless embodiments of a rear suspension system for bicycles that utilizes a swinging pedal crankshaft in U.S. Pat. No. 5,474,318 (1995). A pivotless embodiment (FIG. 8) achieves long-travel with good lateral stiffness by integrating a large x-pivot flexure into its frame. However it is more complicated and expensive and does not have traditional xe2x80x9cdiamond framexe2x80x9d look. The discussion of suspension operation theory in this patent is useful for the understanding of the instant invention and the patent is specifically incorporated by reference.
Other Relevant Art
U.S. Pat. No. 5,452,911 to Klein et al. disclose a chainstay made from highly manipulated tubing and designed to provide high lateral stiffness. However this design is incapable of vertical wheel travel and is known for riding harshly.
A currently popular design uses a solid machined forward chainstay portion, with a tubular rearward portion. For example, a bicycle sold under the trademark Szazbo by Ibis of Santa Rosa, Calif. has a chainstay bridge with triangular apertures designed to provide high lateral stiffness. However designs of this type lack a long, thin, vertically-flexible chainstay, and must rely on one or more pivots to achieve suspension action.
A shock absorbing apparatus for automobile bumpers is disclosed in U.S. Pat. No. 3,856,285 to Yamada. This shows an elastomeric friction element which slides inside a tube to absorb collision energy. A similar shock absorber system for front bicycle suspension is known by the trademark TL5 sold by Interloc of Colma Calif. Neither of these shock absorbers have the capability for angular deflection which is necessary to eliminate all pivots on a bicycle rear suspension. Also, neither shows an external lubrication system for ease of maintenance, or a long friction surface which distributes the frictional forces over a long region for durability and smooth action.
In summary, prior pivoted bicycles give up simplicity and light weight to provide adequate wheel travel. Prior pivotless bicycles typically exhibit problems with low lateral stiffness and inadequate travel. None provides a simple, lightweight, laterally stiff suspension with adequate travel.
Accordingly the objects and advantages of the present invention are:
to provide a bicycle with a rear suspension having improved characteristics;
to provide a simple resilient suspension which eliminates rotating pivot elements while maintaining adequate travel and lateral and torsional rigidity;
to provide a vehicular suspension system with low maintenance requirement, durability, high reliability, low manufacturing costs, a comfortable ride, and light weight to enhance the quality of the riding experience;
to provide a suspension system which is compact, while still providing the advantages of increased travel, such that the system accommodates frames designed to fit riders of shorter stature while maintaining the strength and aesthetic appeal of the traditional diamond frame.
to provide a suspension with progressive springing and damping for high performance yet which does not require pressurized gas or fluid, thereby increasing reliability; and,
to provide a bicycle suspension with a planar truss chainstay structure which is vertically flexible, laterally and torsionally stiff, and easily fabricated.
Still further objects and advantages will become apparent from a consideration of the drawings and ensuing description.
In accordance with the above objectives, and others described herein, the wheel suspension system of the present invention comprises a frame, such as a bicycle frame, with a wheel suspension component for the suspension of a rotatable wheel vertically mounted in the frame. The wheel has a horizontal axle aligned perpendicular to the plane of the wheel. The frame has a triangular wheel suspension component comprising three members (legs of the triangle).
The first member is equivalent to a bicycle chainstay. It has a long axis, a wide lateral side, and a thin vertical side, relative to the lateral side. The first member is adapted to be longitudinally and laterally rigid and vertically flexible. The first member has a wide first end and a bifurcated left/right pair of tapering segments adapted to receive the wheel. The tapering segments have left and right second ends.
The second member is a strut and is equivalent to a bicycle seatstay. It has a long axis, longitudinally expandable and compressible, comprising a unitary top portion with a first end and an integrated shock absorbing means adapted to dampen the longitudinal movement. It also has a bifurcated left/right pair of bottom segments adapted to receive the wheel. The bottom segments have left and right second ends. The bottom left end of the second member is attached to the second left end of the first member and the bottom right end of the second member is attached to the second right end of the first member. A left/right pair of wheel dropouts is integrally located proximal to the second ends of the first member and ends of the bottom segment of the second member and adapted to receive the axle of the wheel.
The rigid third member, equivalent to a bicycle seat tube, has a first and second end wherein the first end is attached to the first end of the first member, and the second end is attached to the first end of the second member.
In various embodiments, one or more junctions between any of the members is rigid or fixed therefore pivotless. In at least one embodiment, all three junctions are pivotless.
In certain embodiments, the shock absorbing means is a long-travel, short-length shock absorber with angular deflection capability. In other embodiments, the shock absorbing means is a commercially-available bicycle shock absorber. Alternatively, an equivalent linkage may function as the shock absorbing means.